This invention relates generally to methods and apparatus for computerized tomographic imaging, and more particularly to methods and apparatus for retrospectively generating computerized tomographic (CT) images of a moving body part without gating signals.
In at least one known computed tomography (CT) imaging system configuration, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system and generally referred to as the "imaging plane". The x-ray beam passes through the object being imaged, such as a patient. The beam, after being attenuated by the object, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is dependent upon the attenuation of the x-ray beam by the object. Each detector element of the array produces a separate electrical signal that is a measurement of the beam attenuation at the detector location. The attenuation measurements from all the detectors are acquired separately to produce a transmission profile.
In known third generation CT systems, the x-ray source and the detector array are rotated with a gantry within the imaging plane and around the object to be imaged so that the angle at which the x-ray beam intersects the object constantly changes. A group of x-ray attenuation measurements, i.e., projection data, from the detector array at one gantry angle is referred to as a "view". A "scan" of the object comprises a set of views made at different gantry angles, or view angles, during one revolution of the x-ray source and detector. In an axial scan, the projection data is processed to construct an image that corresponds to a two dimensional slice taken through the object. One method for reconstructing an image from a set of projection data is referred to in the art as the filtered back projection technique. This process converts the attenuation measurements from a scan into integers called "CT numbers" or "Hounsfield units", which are used to control the brightness of a corresponding pixel on a cathode ray tube display.
For some diagnostic procedures, it is necessary to obtain CT images of a moving body part. For example, cardiac calcification scoring requires CT images of the heart without motion-induced artifacts. One known technique for acquiring CT without motion-induced artifacts is to generate x-rays with a scanning electron beam. The scanning electron beam strikes a metal surface and produces a directed beam of x-rays. The beam of x-rays scan a patient's body so rapidly that motion-induced artifacts resulting from motion during a cardiac cycle are avoided. However, electron beam CT imaging systems are more expensive than CT imaging systems having rotating gantries and are not widely available in all hospitals.
Another known technique for acquiring CT images of a heart is to use EKG gating to select times when a best image of the heart is available. An EKG machine is connected to a patient. A cardiac cycle period is determined, for example, as a time between R-peaks of the EKG. Using an R-peak as a reference and the determined cardiac cycle period, image acquisition during a scan is gated so that image data is acquired only during periods of a cardiac cycle for which the heart is nearly stationary. A disadvantage of this technique is that it requires electronic communication between the CT imaging apparatus and the EKG machine. Furthermore, gating times must be estimated in advance. Unfamiliar surroundings and equipment observed by a patient during a CT scan can induce stress in a patient, resulting in variations of a cardiac cycle during a test. Other anomalies, such as preventricular contractions, may also interrupt a steady cardiac cycle. All of these irregularities reduce the accuracy of the estimated gating times, and can result in unacceptable motion-induced artifacts in the acquired images.
It would therefore be desirable to provide methods and apparatus to reduce or eliminate motion-induced artifacts without requiring expensive equipment, such as an electron-beam CT imaging system, or additional gating signals.